1. Technical Field of the Invention
The present invention relates to a method of forming grids for nuclear fuel assemblies, as well as to the grids formed by the application of the same method.
2. Background Art
In the conventional method of manufacturing nuclear fuel assembly grids, straps 1, which are formed, for example, of nickel alloy so as to have slits therein, are, as shown in FIGS. 7A to 7C, assembled into a grid form, and the intersections 2 of the straps 1 are then brazed. In this method, nickel is usually plated on the surfaces of the straps 1 in advance in order to ensure high-quality brazing.
More specifically, FIG. 7A depicts a part of the assembled straps 1, showing a brazing filler metal (hereinafter referred to as "filler metal") being adherently placed on top of the intersections 2 of the straps 1. When the straps 1 are heated in a vacuum type brazing furnace (hereinafter referred to as "vacuum furnace"), the filler metal melts and flows down along the intersections 2, so that the intersections 2 are presumed to be brazed uniformly over their entire lengths. FIG. 7B depicts the result of brazing in the case where the brazing is effected on the straps having nickel-plated surfaces, whereas FIGS. 7C and 7D depict the result of brazing in the cases where the brazing is carried out on the non-Ni plated straps. In either of the straps shown in FIGS. 7C and 7D, good brazing results have not been obtained due to insufficient or ununiform flow of the filler metal flowing down from the top of the intersections 2 therealong. In FIG. 7C, unbrazed portions are shown remaining, whereas in FIG. 7D, the width and thickness of the brazed parts have become excessive.
Thus, it has hitherto been conventional to carry out nickel plating on the straps 1 in order to ensure excellent brazing quality. However, such a plating process is unavoidably associated with an increased cost and environmental pollution. Therefore, there has been the need for the development of a novel brazing technique which requires no plating treatment but achieves excellent brazing quality.